1. Field of the Invention
This invention relates to thermoplastic polymers having tetrafluoroethylene units and perfluoro alkyl vinyl ether units, mixtures of such polymers that contain low molecular weight and high molecular weight components, and to processes and articles that employ such polymers.
2. Background
Copolymers of tetrafluoroethylene (TFEs below) and perfluoro alkyl vinyl ethers having from 1 to 4 carbon atoms in the alkyl moiety (PAVEs below), in particular perfluoro n-propyl vinyl ether (PPVEs below) have been known for a long time. Such copolymers are commercially available under the designation xe2x80x9cPFAxe2x80x9d. At a PAVE copolymer content of about 2% by weight and greater, these partially crystalline copolymers have excellent technical performance, for example exceptional chemical stability, combined with high service temperatures. They can be processed from the melt as thermoplastics, for example by compression molding, extruding or injection molding. Preferred applications are, inter alia, extruded pipes, tubes and cable sheathing. Processing from the melt takes place at temperatures of from 350 up to 450xc2x0 C. Under these conditions, both thermal and mechanical degradation occur.
The thermal degradation takes place predominantly via the thermally unstable end groups formed in the polymerization, i.e. from the end of the chain. The mechanism of this degradation is described in more detail in xe2x80x9cModern Fluoropolymersxe2x80x9d, John Wiley and Sons, 1997, K. Hintzer and G. Lxc3x6hr, Melt Processable Tetrafluoroethylene-Perfluoropropylvinyl Ether Copolymers (PFA), page 223. The degradation can be substantially suppressed by converting the thermally unstable end groups into stable CF3 end groups by postfluorination, as described, for example in U.S. Pat. No. 4,743,658 and DE-C-19 01 872.
Corrosive gases arise during the thermal degradation, and these considerably impair the quality of the final product by metal contamination or bubble formation, and can corrode tooling and processing machinery, The effect naturally increases with falling molecular weight (lower melt viscosity).
The mechanical degradation during processing takes place through chain breakage, recognizable by the increase of the melt flow index (MFI). It increases as extrusion speed (shear rate) rises. The resultant lowering of molecular weight considerably worsens the mechanical properties, in particular the flexural fatigue strength and other long-term properties, such as long-term failure (stress crack resistance). Keeping the mechanical degradation within acceptable limits places corresponding limitations on processing conditions. This applies in particular to the extrusion speed for pipes, tubes and cable sheathing. At higher extrusion speeds, melt fracture (shark skin) also occurs, as with all thermoplastics. Although it is possible to implement higher extrusion speeds without melt fracture by lowering the molecular weight (higher MFI values), such products do not have adequate mechanical properties. For this reason, PFA products with an MFI value  greater than 15 are currently not on the market.
It is known from WO-A-97/07147 that a marked rise in the extrusion rate is possible, while avoiding melt fracture and with retention of the mechanical properties, with partially crystalline copolymers which consist essentially of TFE and at least 3% by weight of perfluoro ethyl vinyl ether and which have a melt viscosity of not more than 25xc3x97103 Pas at 372xc2x0 C., with the proviso that the melt viscosity may exceed this value if the content of the ether mentioned exceeds 10% by weight. The perfluoro ethyl vinyl ether is, however, difficult to obtain, and therefore all of the marketed products contain PPVE, which is easily obtainable industrially and is also preferred for the present invention.
A PFA has now been found which has good melt processability and which contains at least one high-molecular-weight PFA with an MFIxe2x89xa615, preferably from 0.01 to 15, and at least one low-molecular-weight PFA with MFIxe2x89xa730. The mixtures of the invention are particularly useful in applications where chemical resistance and high temperature resistance are important.
The invention therefore relates to mixtures of thermoplastic fluoropolymers essentially comprising units of TFE and subordinate amounts of units of one or more PAVEs having from 1 to 4 carbon atoms in the alkyl moiety and a total concentration of from 0.5 to 10 mol %, the mixture comprising A) at least one low molecular weight component with an MFIAxe2x89xa730 and B) at least one high molecular weight component with an MFIBxe2x89xa615. These components are selected in such a way that the ratio of the MFIA of component A) to the MFIB of component B) is in the range from 80 to 2500, preferably in the range of from 240 to 750.
xe2x80x9cEssentially comprising units of TFE and of a PAVExe2x80x9d means that small amounts, up to about 5 mol %, of other fluoromonomers not containing hydrogen, such as hexafluoropropene or chlorotrifluoroethylene, are not to be excluded. The composition of the copolymer of the two components may differ within the limits mentioned above.
The mixing ratio of high- and low-molecular-weight components may vary within wide limits and can be determined for the desired application by means of simple preliminary experiments. The ratio is generally from 10:90 to 90:10, preferably in the range from 25:75 to 75:25 parts by weight and in particular from 60:40 to 40:60 parts by weight.
The invention also relates to a novel low-molecular-weight PFA with an MFIxe2x89xa730, preferably xe2x89xa7120 with preference from 120 to 1000, in particular from 120 to 700, especially from 200 to 600.
Another aspect of the invention relates to mixtures of the novel low-molecular-weight PFA(s) mentioned with the high-molecular-weight PFA(s) mentioned above, the MFI ratio mentioned above corresponding approximately to a molecular weight ratio of the high-molecular-weight to the low-molecular-weight component(s) xe2x89xa73.5, preferably from 3.5 to 10, in particular from 3.5 to 7.
The MFI gives the amount of a melt in grams per 10 min which is extruded from a holding cylinder through a die by the action of a piston loaded with weights. The dimensions of the die, the piston, the holding cylinder and the weights are standardized (DIN 53735, ASTM D-1238). All of the MFIs mentioned here have been measured with a die of diameter 2.1 mm and length 8 mm using a superimposed weight of 5 kg and a temperature of 372xc2x0 C. The values 0.01 and 1000 are practically the limiting values of this measurement method.
For very high MFI values, therefore, it is expedient to reduce the superimposed weight to values down to 0.5 kg, and for very small MFI values to increase it to values up to 20 kg. The MFI values determined in this way are recalculated for a superimposed weight of 5 kg.
The present invention further provides a process for making a shaped article from the mixtures of the invention. This process involves providing the mixture, extruding, compression molding, or injection molding the mixture, and preferably, cooling the mixture to provide a self-supporting shaped article.
Still further the present invention provides shaped articles comprising the mixture. Examples of such articles include molded or extruded goods such as films, pellets, wire and cable insulation, tubes and pipes, containers, vessel liners, and the like.